"The problem stems from the fact that most members of Congress represent districts that have become reliably Democratic or Republican; some districts have been gerrymandered that way, but most are now “safe” for one party because of the electorate’s own self-sorting, driven by our growing rural and urban divide. Without any real competition in the general election, the only election of consequence in the large majority of congressional districts––and the only mechanism for accountability––is the primary."
Party primaries make sure that only corporate candidates can win the Dem nominations.